Phytoestrogens, primarily isoflavones and lignans, have been implicated strongly in animal and in vitro experiments as promising compounds against cancer, particularly breast and other hormone-related cancers. Cumulative evidence has indicated that these phytochemicals may interact with estrogens, insulin-like growth factors (IGFs), and pesticides in the etiology of breast cancer. To investigate the role of phytoestrogens and their potential interactions with estrogens, IGFs, and pesticide exposure, we propose to measure urinary phytoestrogens in pre- treatment overnight urine samples collected from a subset (250 case-control pairs) of study participants recruited in the Shanghai Breast Cancer Study, an on-going NCI-funded population- based case-control study among Chinese women in Shanghai. In addition to in-person interviews, fasting blood and urine samples have been collected from over 80 percent of the 3000 women included in the main study. These samples are being used for several ancillary studies, including NCI-funded studies to evaluate the relation of estrogens, IGFs, pesticides, and genetic factors with breast cancer risk, and a NCI contract. For this newly-proposed individually-matched case-control study, urinary levels of all five major isoflavonoids (daidzein, genistein, glycitein, equol, and O-desmethylangolensin) and two main lignans (enterodiol, enterolactone) will be analyzed using HPLC methods. These compounds as well as their potential interactions with estrogens, IGFs, and pesticides will be evaluated in relation to the risk of breast cancer. This study will be unique given the high exposure levels of isoflavones and pesticides and different profile of estrogens and IGFs compared to the western populations. Because data and specimen collection as well as assays for estrogens, IGFs, and pesticides are supported by existing studies, this project will be very cost-efficient. In addition, we will also be able to expand our study to the whole study population if the results are promising. Findings from this study will have important public health implications in breast cancer prevention.